In an ion scattering spectroscopy, a beam of ions of a known fixed kinetic energy is irradiated onto a surface of a solid sample, and the energy spectrum of ions colliding with the atoms in the surface or subsurface and scattered within a preset small solid angle is measured. For measuring the energy of the scattered ions, a method known as the time-of-flight (TOF) method is used.
A TOF type ISS is illustrated with FIG. 7. When a beam of ions 46 is irradiated onto a sample 41, the ions collide with the atoms of the sample 41 and are scattered backward. Some of the scattered ions come into a flight tube 43 through the gate 42. The space within the flight tube 43 is a drift space in which no electromagnetic field is present and the ions fly without being exerted by an external force. Since the speed of an ion is proportional to the square root of its energy, the flight time of an ion in the flight tube 43, thus the arriving time of an ion at an ion detector 40 placed at an end of the flight tube 43, depends on the energy of the ion.
The flight time t.sub.f of an ion having mass m and an initial energy E travelling a distance d is given by EQU t.sub.f =d.multidot.{m/(2.multidot.E)}.sup.1/2. (1)
By measuring the flight time t.sub.f of a scattered ion, its energy E is calculated using the formula (1). When an ion beam injecting onto the sample is pulsated, the starting point of the flight time t.sub.f can be taken from the pulsating timing, and when the injecting beam is continuous, the opening time of the electric field shutter at the gate 42 is used as the starting point of the flight time t.sub.f. In the pulsating (chopping) beam method, therefore, the length of time period from the time when the beam of ions is chopped to the time when the ions scattered by the sample surface arrives to the detector is measured, the speed of scattered ions is calculated, and the mass of atoms of the sample that scattered the ions is calculated.
In such a TOF type ISS, an ion generator is provided in which ions involved in the measurement are generated from a solid or gas ion source. Because the mass of ions used in the measurement should correspond to the sample, proper material of the ion source should be selected depending on the sample. The reason is explained referring to FIG. 8. When an ion of mass m.sub.i running at speed v.sub.1 collides with a stationary atom of mass m.sub.o, the speed v.sub.2 of the ion scattered almost 180.degree. backward is given by EQU v.sub.2 =(m.sub.o -m.sub.i).multidot.v.sub.1 /(m.sub.o +m.sub.i).(2)
If the mass m.sub.i and the speed v.sub.1 of the colliding ion is known, the mass m.sub.o of the object atom can be calculated from the formula (2) by measuring the speed v.sub.2 of the back-scattered ion.
The formula (2) also teaches that the motion of the ion after the collision depends on the mass of the ion m.sub.i and the mass of the atom m.sub.o. When m.sub.o &gt;&gt;m.sub.i, that is the mass m.sub.o of the stationary atom is far larger than the mass m.sub.i of the colliding ion, the speed v.sub.2 of the ion after the collision is almost the same as the speed v.sub.1 before the collision, and the diversity in the speed of scattered ions is so small that the precision of the measurement is low. When, on the other hand, m.sub.o .ltoreq.m.sub.i, that is the mass m.sub.o of the stationary atom is comparable to or smaller than the mass m.sub.i of the colliding ion, the ion cannot be scattered backward and no measurement is possible.
It is necessary therefore to use an ion having a mass slightly smaller than that m.sub.o of the object atom to measure the mass m.sub.o of the atom of the sample precisely. This is why the mass of the ions should be properly selected in the measurement, and the material of the ions source is changed according to the sample. If the object element to be measured is changed, further, though the sample remains the same, the ion source should be changed accordingly. The change of the ion source, albeit solid ion source or gas ion source, requires troublesome operations and long measurement-interrupting time.